The control apparatus according to the present invention finds general utility where one, or a plurality of direct current motors energized by a common power supply must be given a selected predetermined speed-load characteristic in situ for a particular application. It is particularly useful when a plurality of electric motors are required to drive in parallel a common load, as is typical in paper making when a multi-motor drive installation is used. An important requirement in such situation is that each motor share equally or proportionately the load. For load sharing between the motors, a certain similarity between the motor droop characteristics should exist, or if not identical from one motor to the other the motor characteristics must at least be in relation with the imposed different load requirements. Therefore, before starting parallel operation of the motors to be regulated preparatory adjustments have to be made on each motor. In this respect, one way of doing it is to align the motors on the known characteristic of one of them by changing the droop of the others in relation to the desired level of droop. Each motor has an inherent droop, but the latter may be too flat to accommodate the anticipated load changes. Additional droop must be obtained in relation to the increased armature current of the motor when the load increases. For this purpose, the conventional practice has been to add turns to the series field of the machine so that the motor flux increases proportionately to the armature current and thus the speed decreases proportionately. To this end, tests are usually conducted on the test floor in order to provide the required amount of series-field correction.
The use of the series field for such adjustments on the floor is not desirable because it prevents the employ of a standard motor. In addition, large motors are quite expensive to build with a high percentage of droop regulation in this conventional way, since it would take a large series-field winding to do it. If the motor has received such series field, it will be necessary to shunt it in order to get the desired droop.
The object of the present invention is to provide control apparatus for direct current motor drive which is readily adjustable to establish a selected predetermined speed-load characteristic.
Another object of the present invention is to provide direct current motors not requiring any particular series-field winding on the motor therefore.
A further object of the present invention, is to provide group control of parallel direct-current motor drives with individual and electronic adjustment of the motor speed characteristics in relation to load.